Echinodermata (phylum)

Phylum having members that are characterized by a hard, spiny covering or skin. Echinoderms have existed since the Lower Cambrian Period, and today comprise about extant 7,000 species that fall into five well-defined groups: Crinoidea (sea lilies and feather stars), Ophiuroidea (basket stars and brittle stars), Asteroidea (starfishes), Echinoidea (sea urchins, sand dollars, and sea biscuits), and Holothuroidea (sea cucumbers). In the 19th century, Echinodermata was recognized as a distinct group of animals that was occasionally associated with the cnidarians and selected other phyla in a division of the animal kingdom known as the Radiata, however the concept of the superphylum Radiata is no longer valid. Marine invertebrate animals of radially symmetrical structure, having a body wall stiffened by calcareous pieces that may protrude as spines, and tiny hydraulically-operated tube-feet.